The present invention relates generally to broadcast radio signal receiving systems, and more specifically to such systems including a seek function operable to sequentially search a band of radio frequencies for broadcast signals above a predefined signal strength.
Electronic circuits for controlling operation of a broadcast radio receiver are known and have been widely implemented in both home and automotive entertainment systems. One such electronic circuit provides a so-called xe2x80x9cseekxe2x80x9d function operable to sequentially (i.e., either incrementally or decrementally) search a band of radio frequencies for broadcast signals above a predefined signal strength. Typically, such a circuit is responsive to operator selection of the seek function to search either forward or backward from the currently selected broadcast frequency, to identify as an available broadcast station the first broadcast frequency detected as having a signal strength greater than a predefined signal strength, and to tune the radio receiver to the identified broadcast frequency.
Referring to FIG. 1, an example of one known radio frequency receiver 10 is shown having an antenna 12 electronically connected to a tuning circuit 14 via signal path 16. An output of the tuning circuit 12 is connected to an input of a seek circuit 18 via signal path 22, wherein tuning circuit 14 provides an analog radio signal on signal path 22 having a magnitude corresponding to the signal strength of the currently tuned frequency. In this known embodiment, the seek circuit 18 is an analog circuit and includes (among other elements and functions) a comparator circuit 20 having a non-inverting input connected to signal path 22, an inverting input connected to an analog reference voltage VREF, and an output that is fed back to the tuning circuit 14 via signal path 24. A SEEK selector 26 is typically provided on the face plate (not shown) of RF receiver 10 and is electrically connected to seek circuit 18 via signal path 28. The tuning circuit 14 includes another output connected to an audio amplifier circuit (not shown) of RF receiver 10 via signal path 30 as is known in the art.
In normal operation of RF receiver 10, the tuning circuit 14 is operable to tune to a selected frequency, and to provide a corresponding radio signal to the amplifier circuit for audible reproduction of the signal content. During seek operation, in response to operator depression of the SEEK selector 26, seek circuit 18 provides a start signal to tuning circuit 14 via signal path 32. The tuning circuit 14 is responsive to the start signal on signal path 32 to modify, e.g., by either incrementing or decrementing, its tuning frequency and provide a radio signal corresponding thereto to comparator 20 on signal path 22. Comparator 20 is operable to compare the magnitude of the incoming analog radio frequency signal on signal path 22 with the predefined analog reference voltage VREF. As long as the magnitude of the analog radio frequency is less than VREF, the tuning circuit 14 continues to periodically increment or decrement its tuning frequency and pass an analog radio signal corresponding thereto to seek circuit 18. If, however, the magnitude of the analog radio frequency on signal path 22 is greater than VREF, the comparator 20 changes state and provides a stop signal to tuning circuit 14 via signal path 24. The tuning circuit 14 is responsive to the stop signal on signal path 24 to halt the seek process and allow reception by RF receiver 10 of radio signals corresponding to the current tuning frequency. In general, the magnitude of the analog reference voltage VREF in the system of FIG. 1 is typically set at a voltage level above which detected broadcast signals exhibit an acceptable reception quality.
While analog seek circuits of the type illustrated and described with respect to FIG. 1 have been widely used with a number of different antenna configurations (e.g., front mast, rear mast and active backlite antenna systems), they have been found to exhibit certain drawbacks associated therewith. For example, some antenna systems, such as a passive backlite antenna of the type described in co-pending U.S. Pat. No. 6,031,500, which is assigned to the assignee of the present invention and the contents of which are incorporated herein by reference, exhibit significantly lower RF signal gains than conventional antenna systems. While the lower signal gains of such antenna systems generally do not affect the signal-to-noise ratios of the incoming radio frequency signals, they will result in reduced RF signal strengths provided to tuning circuit 14, and accordingly reduced magnitude radio signals provided to seek circuit 18. The quality of reception is mainly dependent upon the signal-to-noise ratio of the received signals and is therefore not affected by lower antenna gains. However, the seek function is dependent only upon the magnitude of the radio signals provided thereto on signal path 22, and therefore is directly affected by the RF signal gain capability of antenna 12. Thus, while lower RF gain antenna systems may have similar quality of reception characteristics across a broadcast frequency band, conventional seek circuitry in receivers utilizing such antennas will typically not detect as many broadcast stations as in receivers utilizing higher RF gain antennas.
One solution to the foregoing drawback associated with low antenna gain RF signal receiving systems is to lower the seek level threshold; i.e., lower the analog reference voltage level VREF. However, while this technique increases the seek count for low RF gain antenna systems, it allows the tuning in of broadcast signals having lower than desired reception quality in systems having higher gain antenna systems. Difficulty accordingly arises when interchanging antenna systems between low and high RF gain antennas. Moreover, circuit changes required to establish different VREF levels for different antenna gains are expensive and difficult to track in a production environment. What is therefore needed is an improved seek threshold strategy that is adaptable to a wide range of antenna RF signal gains and is easily modified in a production environment.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, a radio frequency receiver including a signal seek function comprises a tuning circuit operable in a seek mode to receive and provide radio signals sequentially changing in frequency, means for producing digital signal values corresponding to magnitudes of the radio signals, means for producing a digital reference value corresponding to a reference signal magnitude, and means for producing a stop signal upon detection of a first one of the digital signal values exceeding the digital reference value, wherein the tuning circuit is responsive to the stop signal to halt the seek mode and maintain tuning thereof to a radio signal corresponding to the first one of the digital signal values.
In accordance with another aspect of the present invention, a method of seeking radio frequency signals comprises the steps of conducting a seek process by receiving radio signals sequentially changing in frequency, providing magnitudes of received ones of the radio signals as digital signal values, comparing each of the digital signal values with a digital reference value, and halting the seek process upon detection of a first one of the digital signal values exceeding the digital reference value and allowing reception of radio signals corresponding to the first one of the digital signal values.
One object of the present invention is to provide an improved radio signal seek strategy that is adaptable to a wide range of antenna RF signal gains.
Another object of the present invention is to provide an improved seek threshold circuit that accomplishes these and other objectives.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiment.